1. Technical Field
This invention relates to spacecraft control and, more particularly, to the use of electrochromic devices to control the maneuvering of a spacecraft.
2. Discussion
A difficult problem confronting a spacecraft is that of controlling and stabilizing the orientation of the spacecraft for long periods of time. If the resultant of the solar radiation forces acting on the spacecraft does not pass through the center of mass of the vehicle, a torque that either perturbs the attitude of the vehicle or causes corrective actions by the attitude control system is produced. The result of these attitude corrections is either consumption of propellant or an increment in the momentum stored in attitude control devices such as momentum/reaction wheels or control moment gyros.
In most space projects, solar radiation pressure is still considered primarily as an environmental disturbance source. As early as 1958, solar pressure was also identified as a source of controllable forces applicable to propulsion and attitude control of spacecraft in long duration missions. For example, it has been proposed to provide a trailing cone system to derive control torques from solar radiation. Weathervane-type movable surfaces to develop controllable torques have also been explored. Other researchers have proposed a reflector-collector configuration for using focused radiation, a system of body-fixed corner mirror arrays for pointing the spin axis of a satellite to the sun, and solar paddles for attitude and spin control. Still others considered using a grated solar sail for controlling the attitude of a spinning, sun-orbiting spacecraft. Other approaches considered the application of solar pressure torquing to earth pointing satellites. For example, one approach considered the case of a gravity-gradient-controlled satellite where torques are produced by translation of the control surfaces. In other cases, rotatable control surfaces were used to control the attitude of a dual spin satellite. Time-optimal control of the attitude of a satellite via solar radiation pressure has also been discussed in the literature.
Solar pressure control has been used in various space missions of the past. Mechanically controlled solar vanes were used in the early Mariner spacecraft. Fixed vanes were flown in the OTS satellite and are baselined in several European and future domestic communication satellites for momentum management.
Current techniques for solar pressure trimming, or attitude control by solar pressure, still generally use either a set of fixed, offset panels or electromechanically controlled vanes. These panels or vanes are usually located at the outer ends of the solar arrays or other appendages to maximize the torques attainable. Since the fixed panels must be of relatively large sizes, and placed at large inclination angles with respect to the arrays, provisions for deployment must be made. Deployment facilities add complexity, weight and cost. Movable vanes can be smaller but require hinges or bearings, actuators, and position transducers or sensors.
Therefore, there is a need for an approach to solar pressure trimming and control that is simple, easy to integrate, without moving parts, and has low cost and weight.